Many years ago I had a Krober rev counter on a BSA, this ran off the points via the end cap like the cut out wire, so was using the LT pulse rather than the HT one. Could you couple the meter wire to this instead of the pulse sensor ?
Tried the diode as well as the shielding of the moto gadget leads, still the sameYou should try putting a diode between the moto-gadget sensor and your tachometer. The output from the sensor will be minimal so a small one is OK. ( If you don't have much electronics knowledge a diode is basically a one-way valve.) A 1N4001 diode costs literally pence. Do a search on the internet. If it doesn't work try connecting the diode the other way around as it may be blocking in the wrong direction. There were tachometers that looked like the Smiths ones available a few years ago but the one-man band stopped making them. These worked once a diode was fitted in the feed to the tacho.
Tried the diode as well as the shielding of the moto gadget leads, still the same